Heterogeneous distributions of resources and organisms are characteristic of most ecosystems, but empirical understanding of the causes and consequences of hetero-geneity is limited. We investigated whether predatory fish influenced the heterogeneity (spatial variability) of resources (algae) by modifying the behavior of primary consumers (mayflies). We hypothesized that fish would indirectly increase resource heterogeneity by reducing grazer activity, and that higher densities of grazers would reduce resource het-erogeneity only in the absence of fish. We measured the effects of predator cues (brook trout odor) on grazer behavior (Baetis) and algal heterogeneity in mesocosms (1 m 2) and in simple natural systems (30-m reaches of adjacent fishless streams). In addition, we measured grazer and algal distributions in complex natural streams that varied in grazer density, presence of fish, and physical conditions. Fish odor altered mayfly grazing behavior (measured using a simple behavioral bioassay) and increased algal heterogeneity (measured by Morisita's index at the scale of individual rocks) in mesocosms and manipulated streams. Furthermore, natural streams with higher grazer densities had lower algal heterogeneity, but only if they were fishless. Interestingly, the presence of brook trout decoupled the link between grazer density and algal heterogeneity in natural streams. These observations indicate that release from the threat of predation or increased densities of grazers can homogenize algal resources in fishless streams. We hypothesize that altered foraging in environments with predatory fish, independent of grazer density, led to increased resource patchiness, possibly by allowing the influence of variation in physical characteristics (e.g., flow and substratum) to predominate or by changing grazer microhabitat use. These results support theoretical predictions that factors affecting primary consumer behavior also alter resource heterogeneity.